In the case of conventional organic light-emitting diodes (OLEDs), only some of the generated light is directly coupled out. The rest of the light generated in the active region is distributed to various loss channels, for instance in light which is guided in the substrate, in a transparent electrode and in organic layers by means of wave guide effects, and in surface plasmons which can be generated in a metallic electrode. The wave guide effects occur in particular by reason of the differences in the refractive index at the boundary surfaces between the individual layers and regions of an OLED. In particular, the light which is guided in the loss channels cannot be coupled out from an OLED without additional technical measures.
In order to increase the coupling-out of light and thus the radiated light power, measures are known in order to couple out the light, which is guided in a substrate, into radiated light. For this purpose, films having scatter particles or films having surface structures, such as microlenses, are used, e.g., on the substrate outer side. It is also known to provide direct structuring of the substrate outer side or to introduce scatter particles into the substrate. Some of these approaches, e.g., the use of scattering films, are already used commercially and can be scaled up in particular in the case of OLEDs, which are designed as illumination modules, in relation to the radiating surface. However, these approaches for coupling out light have the significant disadvantages that the coupling-out efficiency is limited to about 60% to 70% of the light guided in the substrate and that the appearance of the OLED is significantly influenced, as a milky, diffusely reflecting surface is produced by the applied layers or films.
Approaches are also known for coupling out the light which is guided in organic layers or in a transparent electrode. However, these approaches have hitherto not become established in a commercial sense in OLED products. For example, the document Y. Sun, S. R. Forrest, Nature Photonics 2,483 (2008) proposes the formation of so-called “low-index grids”, wherein structured regions having a material with a low refractive index are applied onto a transparent electrode. Furthermore, it is also known to apply highly refractive scattering regions under a transparent electrode in a polymeric matrix, as described, e.g., in U.S. patent application 2007/0257608. In this case, the polymeric matrix generally has a refractive index in the region of 1.5 and is applied using wet chemistry. Furthermore, so-called Bragg grids or photonic crystals having periodic scattering structures with structure sizes in the wavelength range of the light are also known, as described, e.g., in the documents Ziebarth et al., Adv. Funct. Mat. 14, 451 (2004) and Do et al., Adv. Mat. 15, 1214 (2003).
OLEDs having a large luminous surface often encounter the problem of light density inhomogeneity as the distance from electrical contacts increases. This problem can be improved by using current-conducting structures, so-called “bus bars” within the active luminous surface. However, structures such as these are visible in the light-emitting pattern of an OLED and for this reason are undesirable.